1. Field of the Invention
The present invention relates generally to a thermal-type flow sensor for measuring a flow rate, for example, of intake air in an internal combustion engine. More particularly, the present invention is concerned with a thermal-type flow sensor for measuring a flow rate (or flow velocity) of a fluid by taking advantage of a phenomenon of heat transfer from a heat generating element (or a part heated by the heat generating element) to the fluid, which sensor can assure an enhanced detection sensitivity and an improved reliability.
2. Description of Related Art
For better understanding of the concept underlying the present invention, description will first be made of conventional thermal-type flow sensors known heretofore by reference to figures. FIG. 18 shows in a top plan view a bridge type flow rate detecting device 18 employed in a conventional thermal-type flow sensor disclosed, for example, in Japanese Patent Publication No. 7659/1993, wherein the flow rate detecting device 18 is shown in a state where a protection film 3 is removed, and FIG. 19 shows a side-elevational sectional view of the same taken along a line X--X in FIG. 18.
Referring to FIGS. 18 and 19, a plate-like substrate 1 is made of a silicon semiconductor material.
The thermal-type flow sensor is further composed of a supporting or base film 2 and the protection film 3 each formed of an insulative silicon nitride material deposited over a whole top surface of the substrate 1. A heat generating resistor pattern 4 deposited on the base film 2 is formed of a heat-sensitive resistance film such as of permalloy, platinum or the like. In this conjunction, the term "heat-sensitive resistance film" means a resistant film formed of a material whose resistance value exhibits a temperature dependency.
Temperature measuring resistor patterns 5 and 6 also deposited on the base film 2 are each formed of a heat-sensitive resistance film similarly to the heat generating resistor pattern 4. The temperature measuring resistor patterns 5 and 6 are disposed, respectively, at both sides of the heat generating resistor pattern 4 on a same plane as the latter. More specifically, the temperature measuring resistor patterns 5 and 6 are juxtaposed in a planar array in a fluid flow direction (indicated by an arrow G in FIG. 18) with the heat generating resistor pattern 4 being interposed therebetween.
A reference resistor pattern 7 also deposited on the base film 2 is formed of a heat-sensitive resistance film similarly to the patterns mentioned above and deposited or disposed on a same plane as the heat generating resistor pattern 4 and the temperature measuring resistor patterns 5 and 6.
The heat generating resistor pattern 4, the temperature measuring resistor patterns 5 and 6 and the reference resistor pattern 7 are incorporated in a control circuit of a thermal-type flow sensor in the manner well known in the art, although illustration thereof is omitted.
More specifically, the reference resistor pattern 7 constitutes a bridge circuit through cooperation with the temperature measuring resistor patterns 5 and 6, wherein a constant voltage is applied across the bridge circuit from the control circuit. On the other hand, a heating current is fed to the heat generating resistor pattern 4 from the control circuit, whereby a voltage making across the heat generating resistor pattern 4 and corresponding to the heating current is outputted as a flow-rate measurement signal.
A pair of openings 8 are formed in the vicinity of the array or region of the heat generating resistor pattern 4 and the temperature measuring resistor patterns 5 and 6 at upstream and downstream sides thereof, wherein the pair of openings 8 are communicated to each other through an air space 9.
The air space 9 is formed by removing partially the silicon semiconductor material through the openings 8 by using a liquid-phase etchant which does not exert any adverse influence to the silicon nitride film.
In this manner, the array composed of the heat generating resistor pattern 4 and the temperature measuring resistor patterns 5 and 6 forms a bridge portion 11 (low heat capacity portion).
Next, description will be directed to operation of the conventional thermal-type flow sensor in which the flow rate detecting device 18 shown in FIGS. 18 and 19 is employed.
The heating current supplied to the heat generating resistor pattern 4 from the control circuit (not shown) is so controlled that the heat generating resistor pattern 4 can be heated to a predetermined temperature which is higher, for example, by 200.degree. C. than the temperature of the plate-like substrate 1 which is detected by the reference resistor pattern 7.
Heat generated by the heat generating resistor pattern 4 is transferred to the temperature measuring resistor patterns 5 and 6 by way of the base film 2 and the protection film 3 and/or other heat-sensitive resistance film(s), if present.
In this conjunction, it is to be noted that the temperature measuring resistor patterns 5 and 6 are disposed at respective positions symmetrically to each other with reference to the heat generating resistor pattern 4. Accordingly, so long as no fluid flow exists, there will arise no difference in the resistance value between the temperature measuring resistor patterns 5 and 6.
By contrast, when fluid flow such as air flow exists on and along the temperature measuring resistor patterns 5 and 6, the temperature measuring resistor pattern located at the upstream side as viewed in the fluid flow direction is cooled by the air, while the temperature measuring resistor pattern located at the downstream side is not cooled to a same extent as the temperature measuring resistor pattern positioned at the upstream side, because the downstream temperature measuring resistor pattern is less susceptible to the influence of heat transferred from the heat generating resistor pattern 4 to the air when compared with the upstream temperature measuring resistor pattern.
By way of example, it is assumed that the air flow takes place in the direction indicated by the arrow G in FIGS. 18 and 19. Then, the temperature of the upstream temperature measuring resistor pattern becomes lower than that of the downstream temperature measuring resistor pattern 6. In general, difference in the resistance value between the temperature measuring resistor patterns 5 and 6 increases as the flowing velocity or flow rate of the fluid (air) becomes high.
Thus, by detecting the resistance values of the temperature measuring resistor patterns 5 and 6, respectively, it is possible to measure the flowing velocity or the flow rate of the air.
Such measurement of the flow rate can equally be performed even in the case where the air flows in the direction opposite to that indicated by the arrow G because then the temperature of the temperature measuring resistor pattern 6 becomes lower than that of the temperature measuring resistor pattern 5. Besides, with the arrangement of the heat generating resistor pattern 4 and the temperature measuring resistor patterns 5 and 6, the fluid flow direction can also be detected.
The foregoing description has been made of the flow rate detecting device 18 which includes the bridge portion 11 as the low heat capacity portion. It is however to be mentioned that a wide variety of flow rate detecting devices have also been proposed in which a diaphragm, for example, is employed as the low heat capacity portion.
FIG. 20 is a top plan view showing a diaphragm type flow rate detecting device 18a employed in a conventional thermal-type flow sensor, wherein the flow rate detecting device 18a is shown in a state where a protection film is removed, and FIG. 21 is a side-elevational sectional view of the same taken along a line Y--Y in FIG. 20. In FIGS. 20 and 21, components same as or equivalent to those mentioned hereinbefore by reference to FIGS. 18 and 19 are denoted by like reference characters, and detailed description thereof will be omitted.
As can be seen in FIGS. 20 and 21, the plate-like substrate 1 has a cavity 12 which is formed by removing partially the material of the plate-like substrate 1 through an etching process from the side opposite to the surface of the plate-like substrate 1 on which the base film 2 is deposited.
Thus, the base film 2 and the protection film 3 between which the heat generating resistor pattern 4 and the temperature measuring resistor patterns 5 and 6 are sandwiched cooperate to constitute a diaphragm 13, as can clearly be seen in FIG. 21.
Parenthetically, it should be mentioned that the diaphragm type flow rate detecting device 18a shown in FIGS. 20 and 21 can ensure higher mechanical strength when compared with the bridge type flow rate detecting device 18 described hereinbefore by reference to FIGS. 18 and 19.
Thus, the diaphragm type flow rate detecting device 18a is suited for use under severe environmental conditions such as encountered in detecting the intake-air flow rate in an engine for a motor vehicle. Incidentally, in the diaphragm type flow rate detecting device 18a, the principle of detecting the flow rate (or flowing velocity) of the air is essentially same as that incarnated in the bridge type flow rate detecting device 18 described hereinbefore.
As another one of the thermal-type flow sensors known heretofore, there may be mentioned an indirect heating type flow sensor in which a heat measuring element is disposed at an upstream position as viewed in the fluid flow direction with a heating element being disposed at a downstream position in order to realize output improved characteristics exhibiting high linearity as a function of the flowing velocity, as is disclosed, for example, in Japanese Unexamined Patent Application Publication No. 174600/1995 (JP-A-7-174600).
Of the thermal-type flow sensors mentioned above, the thermal-type flow sensor which incorporates the bridge type flow rate detecting device 18 (shown in FIGS. 18 and 19) can not assure sufficiently high mechanical strength when compared with the thermal-type flow sensor in which the diaphragm type flow rate detecting device 18a (shown in FIGS. 20 and 21) is employed, because the area over which the bridge portion 11 is physically supported by the plate-like substrate 1 is smaller in the case of the bridge type flow rate detecting device 18.
Accordingly, in order to realize a sufficiently high mechanical strength in the bridge type flow rate detecting device 18, it is required to increase the film thickness of the bridge portion 11 as a whole or increase the area over which the bridge portion 11 is brought into contact with the plate-like substrate 1. In that case, however, the overall size of the thermal-type flow sensor will increase, to a disadvantage.
On the other hand, the thermal-type flow sensor in which the flow rate detecting device 18a is employed is certainly advantageous in respect to the mechanical strength as compared with the flow sensor having the bridge type flow rate detecting device 18. However, in the case of the former, the diaphragm 13 is brought into contact with the plate-like substrate 1 along the whole circumstance of the diaphragm 13. Consequently, an increased amount of heat generated by the heat generating resistor pattern 4 will be transferred to the plate-like substrate 1 by way of the diaphragm 13, which results in degradation of the flow-rate detection sensitivity of the thermal-type flow sensor, giving rise to a problem.
It is further noted that in the flow rate detecting device 18a incorporating the diaphragm 13 contacted to the plate-like substrate 1 over the whole circumference of the diaphragm, it is difficult to enlarge the area of the heat generating resistor pattern 4, which means that the heat transfer rate can not be increased, thus incurring a problem that the flow-rate detection sensitivity is low.
Certainly, the detection sensitivity of the flow rate detecting device 18a can be protected against degradation to some extent by forming the diaphragm 13 as large and thin as possible. In that case, however, the strength of the diaphragm 13 becomes low, presenting a problem in practical applications.
By the way, in the measurement, for example, of the intake-air flow rate in an internal combustion engine, an anti-dust filter is ordinarily disposed at a position upstream of the thermal-type flow sensor. However, fine dust particles and moisture can pass through the filter to be deposited on the flow rate detecting device, as known by those skilled in the art.
Deposition of the dust and the moisture mentioned above will of course be accompanied with contamination of the flow rate detecting device, which will bring about variation or change more or less in the heat transfer characteristic between the flow rate detecting device and the air flow, as a result of which drift may undesirably occur in the flow-rate detection characteristics.
Such being the circumstances, it is also known heretofore to suppress the drift of the flow-rate detection characteristics of the sensor by burning the dusts while gasifying the moisture by increasing the temperature of the flow rate detecting device in order to cope with the contamination of the flow rate detecting device.
However, because the ambient temperature of the heat generating resistor pattern 4 becomes lower as the distance from the heat generating resistor pattern 4 increases, and when the temperature measuring resistor is disposed at the upstream side of the heating element with a view to improving the detection sensitivity, as disclosed in Japanese Unexamined Patent Application Publication No. 174600/1995, the dust and moisture deposited on the temperature measuring resistor can not be removed because the temperature of the temperature measuring resistor is lower than that of the heating element. Thus, degradation will be involved in the anti-contamination performance of the flow sensor.
On the other hand, when the temperature of the temperature measuring resistor is set high by setting high the temperature of the heating element in order to protect the flow rate detecting device against degradation of the anti-contamination performance, the diaphragm 13 may possibly be impaired or damaged under the adverse influence of the heat because the temperature of the heating element disposed downstream of the temperature measuring resistor must be set high particularly when the flow rate is high, incurring thus degradation of reliability of the operation of the thermal-type flow sensor.
More specifically, assuming that the temperature of the heating element is set at a predetermined level capable of preventing the contamination, then the temperature of the temperature measuring resistor is necessarily lower than that predetermined level, as a result of which contamination of the temperature measuring resistor will occur. On the other hand, when the temperature of the temperature measuring resistor is set at the aforementioned predetermined level, then the temperature of the heating element has to be set higher than that predetermined level mentioned above, which will involve damage of the diaphragm.
It is further noted that in the case where the thermal-type flow sensor is employed as an intake air flow sensor in a vehicle-onboard internal combustion engine for effectuating the fuel control, a sum of the flow rate in the reverse flow direction and that in the forward flow direction will be detected as the intake-air flow rate in the state where pulsating flow phenomenon accompanied with the reverse flow of the intake air is taking place in an operation range where the throttle opening degree is set large (known as the valve overlap operation region). Thus, flow-rate detection error twice as large as the flow rate in the reverse direction may occur.
As is apparent from the foregoing description, the conventional thermal-type flow sensor in which the bridge type flow rate detecting device 18 is employed suffers a problem that sufficient strength can not be ensured because the area over which the bridge portion 11 is supported on the plate-like substrate 1 is small.
On the other hand, the hitherto known thermal-type flow sensor in which the diaphragm type flow rate detecting device 18a is employed is disadvantageous in that because the diaphragm 13 is in constant with the plate-like substrate 1 along the whole peripheral portion, the amount of heat conducted to the plate-like substrate 1 from the heat generating resistor pattern 4 by way of the diaphragm 13 is large and that attempt for increasing the area of the heat generating resistor pattern 4 encounters difficulty in practical application, as a result of which high flow-rate detection sensitivity can not be realized.
Further, when the temperature measuring resistor is disposed at the upstream side of the heating element (heat generating resistor pattern), as disclosed in Japanese Unexamined Patent Application Publication No. 174600/1995, the dust and moisture deposited on the temperature measuring resistor can not be removed because the temperature of the temperature measuring resistor is lower than the temperature of the heating element. Thus, degradation will be involved in the anti-contamination performance, giving rise a problem.
In conjunction with the flow sensor mentioned just above, it is further noted that when the temperature of the temperature measuring resistor is set to a higher level in an effort to protect the anti-contamination performance against degradation, the temperature of the heating element disposed downstream of the temperature measuring resistor becomes excessively high, giving rise to anther problem that the thermal reliability is lowered.
Moreover, it is noted that in the case where the conventional flow rate detecting device is employed in the intake air flow sensor for the vehicle-onboard internal combustion engine, the flow rate in the reverse flow direction upon occurrence of the pulsating flow phenomenon will be detected intactly as that in the forward flow direction. Thus, flow-rate detection error substantially twice as large as the reverse flow rate may occur, to a further problem.